Martian Volcanic Plumes Shorter than Previously Thought

Martian volcano Olympus Mons, a shield volcano produced by lava flows. (Credit: NASA)

The maximum height of explosive volcanic eruption columns on Mars dictates how far ash could be transported across the planet surface. The higher a volcanic plume rises, the farther ash can be deposited. When ash is being transported in the atmosphere, it is sorted based on size and density; the resulting deposits can look very similar to the sorted material produced by flowing water. Thus, it is vital to constrain which deposits could have been produced by a volcano and which are more likely to have been water-lain when contemplating future Mars Rover landing sites. Previous models have predicted that plumes could rise more than 100 km into the atmosphere, which means that deposits found over 100 km away from the vent could have been caused by a volcano, not water.

Martian plume models are based off terrestrial plume models, developed through laboratory experiments on Earth. These models have several boundary assumptions, including that the rise velocity and expansion rate of the plume are slower than the speed of sound, the expansion rate is less than the rise rate, and the radius of the plume is not larger than the height of the plume. The largest observed terrestrial eruption columns do not violate these conditions, signaling model appropriateness for terrestrial research. To make these models applicable to Mars, scientists changed the Earth specific variables, such as atmospheric conditions and gravity, to reflect those on Mars. However, there is not a compelling reason to believe that the assumptions underlying these models are equally translatable for Martian conditions.

Plume rise velocity vs. altitude. Note that the rise speed exceeds the speed of sound at ~40 km altitude for this example. Different eruption conditions resulted in different heights for the speed of sound violation.  (Figure 5 from Glaze and Baloga, 2002)

A 2002 study assessed maximum Martian plume heights by testing whether the model assumptions were valid. After looking at the model’s physics, which remains the same whether on Earth or on Mars, they found that the source conditions that produced the largest plumes on paleo Mars violated all of these assumptions. They found that the vertical velocity exceeds the speed of sounds at heights 4 times shorter than previously thought. They also found that after 50 km of rise, the radius of the plume is more than 10 times wider than the height! Additionally the radial velocity exceeds the speed of sound at heights 5 times shorter that the maximum plume height. The radial velocity is faster than the rise velocity nearly 7 times faster than previously thought, due to a much lower atmospheric density. When the atmospheric density is lowered, the expansion rate does not change by the same amount. For example, if atmospheric density is lowered by a factor of 2, the expansion rate does not decrease by a factor of 2. The result of all of this violation of model assumptions is that we cannot fully trust the existing plume models to describe eruptions on Mars.

Based on the limitations of the existing models, the authors suggest that the maximum plume height for eruptions on paleo-Mars be considered as 65 km, the maximum height before the model broke down. This decreased maximum plume height similarly decreases the maximum extent expected from associated fall deposits. Further work linking ash transport to plume heights in Martian atmosphere, as well as transport effects from eruption column collapse, can help Earth-bound researchers identify which deposits on the Martian surface are most likely to have been lain by water by ruling out volcanic sources.

Original paper: Glaze, L. S., and S. M. Baloga, Volcanic plume heights on Mars: Limits of validity for convective models, J. Geophys. Res., 107(E10), 5086, doi:10.1029/2001JE001830, 2002.